Spinal fixation rod made of titanium alloy

ABSTRACT

A spinal fixation titanium alloy rod fixes a plurality of spinal-fixing screws embedded and fixed in vertebrae of a human body. The rod is cylindrically shaped, has a sufficient length for coupling with the spinal-fixing screws, and has a diameter adjusted to 4 to 7 mm. In the titanium alloy constituting the rod, Nb content is 25 to 35 percent by weight, Ta content is such that the Nb content +0.8×Ta content ranges from 36 to 45 percent by weight, Zr content is 3 to 6 percent by weight, and the remainder is Ti and unavoidable impurities, excluding vanadium. The titanium alloy is manufactured by swaging processing at a cross-sectional reduction rate of at least 90%, and aging the swaged titanium alloy by heating at a temperature of 600 to 800K, preferably 700 to 800K, for 43.2 ks to 604.8 ks.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application and thus claims benefitpursuant to 35 U.S.C. §121, of U.S. patent application Ser. No.12/965,424 filed Dec. 10, 2010, which claims priority to Japanese PatentApplication No. 2010-132886 filed on Jun. 10, 2010, in Japan, thecontents of which are incorporated herein by reference.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention generally relates to a spinal fixation rod made oftitanium alloy rod for coupling with and fixing screws used for fixingthe spine.

2. Background Art

Typically, a spinal fixation appliance is utilized as a system assembledfrom various types of parts and is prepared with a size and shapeaccording to the intended use. A typical example suitable as a spinalfixation appliance is a spinal fixation system for fixation of thevertebrae by fixing a plurality of screws embedded in the vertebrae.

The spinal fixation rod is normally shaped as a 4 to 7 mm diametercylinder having sufficient length for coupling with and fixing theplurality of screws embedded in the vertebrae. During an operation,while the physician elastically deforms the rod to match the curvatureof the vertebrae of the spine of the human body, the rod for couplingand fixing the plurality of screws embedded in the vertebrae is coupledto a receiving part of the head part of the screws, and then a rodapproximetor is used to fix the rod to the screw heads using set screws.

Providing the rod with biocompatibility is important, because the rod isused by embedding the rod for a long time within the human body. Inaddition to the rod needs to be capable of being bent by the physicianin order to be deformed during the operation to match curvature of thevertebrae of the spine, the rod also preferably does not lose toughnessat the bent part. Conversely, a spinal fixation rod embedded in thehuman body may be subjected to large loads from various angles due tobending and stretching of the vertebrae associated with movement of thehuman body. Therefore, the rod is required to resist elastic deformationdue to a load. However, stress shielding occurs when the spinal fixationappliance is excessively hard in comparison to the bones of the humanbody. Therefore, the rod preferably has an elastic modulus similar tothat of human bone. The rod absolutely must have a fatigue strength ofat least a certain value in order to be used long term within the humanbody. In order for the rod to be capable of use as a spinal fixationrod, the material constituting the rod is required to simultaneouslysatisfy such mutually contradictory requirements as described above.

Although stainless steel, Co—Cr alloy, pure titanium, and Ti alloy areknown as materials for use in a spinal fixation rod, finding a materialthat satisfies, with good balance, the above-described severe conditionsrequired for a spinal fixation rod is not easy. Among theabove-described materials, Ti alloys have recently been frequently usedas materials for spinal fixation rods because of their extremely highsafety due to resistance to corrosion in the human body and goodcompatibility with tissues.

Ti alloy spinal fixation rods have been manufactured heretofore asappliances made from Ti-6Al-4V (mass %) alloy, and such spinal fixationrods are used most often due to resistance to elastic deformation due toload (permanent deformation) or the like. However, such Ti alloy spinalfixation rods have been indicated to have problems such as the readyoccurrence of stress shielding, because the Ti-6Al-4V alloy has a highelastic modulus in comparison to human bone, vanadium capable of harmingthe human body is included, and the like.

Under such circumstances, a spinal fixation rod and a spinal fixationsystem using such a spinal fixation rod are presently needed in order toovercome all the deficiencies of the spinal fixation rods using theTi-6Al-4V alloy. The spinal fixation rod and spinal fixation systemshould have characteristics meeting, with good balance, the variousrequirements for a spinal fixation rod during a spinal fixationoperation.

SUMMARY OF INVENTION

An aspect of the present invention is a spinal fixation rod made oftitanium alloy for fixing a plurality of spinal fixation screws embeddedand fixed in vertebrae of a human body. The rod is cylindrically shapedhaving a diameter adjusted to 4 to 7 mm and has a sufficient length forcoupling with the spinal fixation screws. The titanium alloy comprisesNb content of 25 to 35 percent by weight, Ta content of such that the Nbcontent +0.8×Ta content ranges from 36 to 45 percent by weight, Zrcontent of 3 to 6 percent by weight, and the remainder is Ti andunavoidable impurities, excluding vanadium. The rod is produced bysubjecting a cylindrical rod made of the titanium alloy to a swagingprocessing of cross-sectional reduction rate of at least 90%, and agingthe swaged rod by heating at a temperature of 600 to 800K, preferably700 to 800K, for 43.2 ks to 604.8 ks.

Another aspect of the present invention is a spinal fixation titaniumalloy rod for fixing a plurality of spinal fixation screws embedded andfixed in vertebrae of a human body. The titanium alloy comprises Nbcontent of 25 to 35 percent by weight, Ta content of such that the Nbcontent +0.8×Ta content ranges from 36 to 45 percent by weight, Zrcontent of 3 to 6 percent by weight, and the remainder is Ti andunavoidable impurities, excluding vanadium. The titanium alloy rod hasproperties under JIS of: a) tensile strength is greater than or equal to1,150 MPa, b) fatigue strength is greater than 900 MPa, c) elasticmodulus is less than 110 GPa, d) 0.2% proof stress is greater than 1,000MPa, and e) percentage elongation after fracture is greater than orequal to 15%.

Another aspect of the present invention is a spinal fixation rod made oftitanium alloy for fixing a plurality of spinal-fixing screws embeddedand fixed in vertebrae of a human body. The rod is produced bysubjecting a cylindrical rod made of titanium alloy to a swagingprocessing of a cross-sectional reduction rate greater than or equal to90% wherein the titanium alloy comprises Nb content of 25 to 35 percentby weight, Ta content of such that the Nb content +0.8×Ta content rangesfrom 36 to 45 percent by weight, Zr content of 3 to 6 percent by weight,and the remainder is Ti and unavoidable impurities, excluding vanadium,aging the swaged titanium alloy by heating at a temperature of 700 to800K for 43.2 ks to 604.8 ks; and machining the aged titanium alloy intoa cylindrical shape having a diameter of 4 to 7 mm.

Another aspect of the present invention is method for producing a spinalfixation rod made of titanium alloy for fixing a plurality of spinalfixation screws embedded and fixed in vertebrae of a human body. Themethod comprises: subjecting a cylindrical rod made of the titaniumalloy to a swaging processing of a cross-sectional reduction rategreater than or equal to 90% wherein the titanium alloy comprises Nbcontent of 25 to 35 percent by weight, Ta content of such that the Nbcontent +0.8×Ta content ranges from 36 to 45 percent by weight, Zrcontent of 3 to 6 percent by weight, and the remainder is Ti andunavoidable impurities, excluding vanadium; aging the swaged titaniumalloy rod by heating at a temperature of 700 to 800K for 43.2 ks to604.8 ks; and precision machining the aged titanium alloy rod into acylindrical shape having a diameter of 4 to 7 mm.

Another aspect of the present invention is a spinal fixation system. Thesystem comprises: a plurality of screws made of titanium alloy; a spinalfixation rod manufactured by steps comprising: subjecting a cylindricalrod made of titanium alloy to a swaging processing of a cross-sectionalreduction rate greater than or equal to 90% wherein the titanium alloycomprises Nb content of 25 to 35 percent by weight, Ta content of suchthat the Nb content +0.8×Ta content ranges from 36 to 45 percent byweight, Zr content of 3 to 6 percent by weight, and the remainder is Tiand unavoidable impurities, excluding vanadium; and aging the swagedtitanium alloy by heating at a temperature of 600K to 800K, preferably700K to 800K for 43.2 ks to 604.8 ks; and a plurality of set screws forfixing the plurality of screws with the rod, the set screws being madeof stainless Ti-6Al-4V alloy.

Another aspect of the present invention is a method for connecting aplurality of screws that are screwed into a plurality of vertebrae. Themethod comprises: screwing a plurality of the spinal fixation screwsinto the plurality of vertebrae, the screw having a rod receiver at atop part; placing a spinal fixation rod within the receiver, the rodbeing produced by: subjecting a cylindrical rod made of the titaniumallow to a swaging processing of cross-sectional reduction rate greaterthan or equal to 90% wherein the titanium alloy comprises Nb content of25 to 35 percent by weight, Ta content of such that the Nb content+0.8×Ta content ranges from 36 to 45 percent by weight, Zr content of 3to 6 percent by weight, and the remainder is Ti and unavoidableimpurities, excluding vanadium; and aging the swaged titanium alloy byheating at a temperature of 600 to 800K for 43.2 ks to 604.8 ks; andfixing the rod with the screw using a rod approximetor.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a tilted perspective drawing of a spinal fixation applianceusing a spine fixation titanium alloy rod according to one or moreembodiments of the present invention.

FIG. 2 shows a figure showing results of optical microscopy structureobservations for the rods of examples 1 and 2 according to one or moreembodiments of the present invention, and comparative examples 2, 3, and4.

FIG. 3 shows is a figure showing results of FE-SEM structureobservations for the rods of examples 1 and 2 according to one or moreembodiments of the present invention, and comparative examples 3 and 4.

FIG. 4 shows results of measurement of the Young's modulus by the freeresonance method for the rods of examples 1 and 2 according to one ormore embodiments of the present invention, and comparative examples 1,2, 3, 4, and 7.

FIG. 5 shows results of measurement of tensile testing for the rods ofexamples 1 and 2 according to one or more embodiments of the presentinvention, and comparative examples 1, 2, 3, 4, and 7.

FIG. 6 shows results of measurement of fatigue testing for the rods ofexamples 1 and 2 according to one or more embodiments of the presentinvention, and comparative examples 1, 2, 3, 4, and 7.

FIG. 7 shows results of measurement of tensile strength, 0.2% proofstress, and elongation for the TNTZ alloy according to one or moreembodiments of the present invention.

FIG. 8 shows results of measurement of tensile strength, 0.2% proofstress, and elongation for the TNTZ alloy according to one or moreembodiments of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention are explained below, referring tothe attached figures. In embodiments of the invention, numerous specificdetails are set forth in order to provide a more thorough understandingof the invention. However, it will be apparent to one of ordinary skillin the art that the invention may be practiced without these specificdetails. In other instances, well-known features have not been describedin detail to avoid obscuring the invention.

FIG. 1 is a tilted perspective view of the spinal fixation appliance 1using a spinal fixation titanium alloy rod 2 of the present invention.As shown in FIG. 1, the spinal fixation appliance 1 has a spinalfixation rod 2 and a screw 3. By fixing both tip sides of the spinalfixation rod 2 through the set screws 6 to the head parts 4 of thescrews 3 embedded in the vertebrae 5, the vertebrae 5 are fixedtogether. The spinal fixation rod 2 in this manner is used as part ofthe spinal fixation appliance 1 for fixing the spine.

In the titanium alloy used in the spinal fixation titanium alloy rod inone or more embodiments of the present invention, the Nb content is 25to 35 percent by weight, Ta content is such that the Nb content +0.8×Tacontent ranges from 36 to 45 percent by weight, Zr content is 3 to 6percent by weight, and the remainder is Ti and unavoidable impurities,excluding vanadium (hereinafter, this Ti alloy composition will beabbreviated as the “TNTZ alloy”).

The above-described round rod formed from TNTZ alloy can bemanufactured, for example, by levitation melting of a powder of theabove-described various metals at the above-described weight percentconcentrations, thereafter casting to form an ingot, and, then, hotforging to produce a round rod of about 24 mm diameter. This roundrod-shaped casting can be subjected to solution heat treatment. Suchsolution heat treatment, for example, can be performed by heating theround rod for 0.6 ks to 3.6 ks at a temperature of 973 to 1,073K andthen water cooling the heat treated round rod. A method for themanufacture of TNTZ alloy is disclosed in Japanese Laid-Open UnexaminedPatent Application 2002-18016, which is incorporated by reference aspart of the present application.

The above-described 24 mm diameter Ti alloy round rod is swaged at across-sectional reduction rate of at least 90% to produce a round rod of7 mm diameter. In this context, swaging refers to beating while rotatingthe round rod to cold work and reduce diameter of the round rod. Whenswaging of an α+β phase Ti alloy (e.g. Ti-6Al-4V) is performed so thatthe cross-sectional reduction rate is greater than or equal to 90%,cracks form in the workpiece or the workpiece breaks. However, TNTZalloy has β phase as a mother phase and, thus, there is no concern thatcracking or breaking will occur even when the workpiece is subjected toextreme swaging (i.e. greater than or equal to 90%).

Immediately after the above-described swaging, the TNTZ alloy issubjected to aging treatment. During the aging treatment, the heatingtemperature is set in the range of 600K to 800K, preferably 700K to800K, and the TNTZ alloy is heated for 43.2 ks to 604.8 ks.

Although it is not necessarily clear what chemical changes occur in theTNTZ alloy due to the above-described swaging and subsequent agingtreatment, it has been found by the inventors of the present inventionthat a large amount of dislocations are introduced into the grains ofthe β phase rather than there being a change in the crystal structure (βphase) of the TNTZ alloy. It is believed that a large amount of the finea phase precipitates within the β phase grains due to the heating duringthe post-swaging aging treatment.

The titanium alloy rod manufactured in this manner has the mechanicalproperties of an elastic modulus (Young's modulus) ranging between 60GPa and 110 GPa, an tensile strength greater than 1,150 MPa, a fatiguestrength greater than 900 MPa, a 0.2% proof stress greater than 1,000MPa, and an elongation greater than or equal to 15%. Tensile strength,0.2% proof stress (proof stress calculated by off set method) andpercentage elongation after fracture are measured in accordance with JISZ 2241 (method of tensile test for metallic materials) of 1993 version.Fatigue strength is measured in accordance with the method described inJIS Z 2273 (General rules for fatigue testing of metals) of 1974.Young's modulus is measured in accordance with the method described inJIS Z 2280 (Test method for Young's modulus of metallic materials atelevated temperature) of 1993. Disclosure of these JIS is incorporatedby reference as part of the present application.

The Young's modulus of the TNTZ alloy manufactured in theabove-described manner is low in comparison to Ti-6Al-4V alloy. Thus,when TNTZ alloy spinal fixation rod is implanted in the human body,there is less concern that stress shielding may occur in comparison tothe rod manufactured from Ti-6Al-4V alloy.

Fatigue strength of the TNTZ alloy rod manufactured in theabove-described manner is greater than 900 MPa. This value is excellentin comparison to the fatigue strength of the rod manufactured from theconventional Ti-6Al-4V alloy, fatigue strength of which is about 725MPa. According to findings of the inventors of the present invention,due to swaging and the post-swaging aging treatment, the α phaseprecipitates within the grains of the β phase (mother phase) of the TNTZalloy, and this is believed to stop the promotion of fatigue cracks bythe α phase.

Proof stress (0.2% proof stress) of the TNTZ alloy rod manufactured inthe above-described manner, which is measured as proof stress calculatedby offset method in accordance with JIS Z 2241 of 1993, ranges from1,000 MPa to 1,300 MPa. When the proof stress ranges from 1,000 MPa to1,300 MPa, the physician during the operation is able to plasticallydeform the rod to match curvature of the spine of the patient.

Percentage elongation after fracture of the rod manufactured from TNTZalloy becomes greater than or equal to 15% due to the above-describedpost-swaging aging treatment for 3.2 ks to 604.8 ks at a temperature ofat least 700K. When the post-swaging aging treatment is performed at atemperature somewhat lower than 700K (e.g. 673K), although the obtainedTNTZ alloy rod has excellent tensile strength and 0.2% proof stress,elongation is low, i.e. about 11%. When the post-swaging aging treatmentis instead performed at a temperature of at least 700K (e.g. 723K),excellent physical properties are obtained, including percentageelongation after fracture of at least 15%. When swaging and agingtreatment are performed under such high temperature, TNTZ alloy rodtensile strength and proof stress (0.2% proof stress) are not lost.

When the spinal fixation rod is plastically deformed by bending by thephysician during the operation, toughness of the bend part may be lostdue to such bending deformation, and such loss of toughness may beundesirable for a rod implanted in the human body. When percentageelongation after fracture of the TNTZ alloy used in the spinal fixationrod is greater than or equal to 15%, bending of the rod by the physicianduring the operation does not cause toughness of the bent part to belost after plastic deformation. Thus, if the spinal fixation rodsatisfies other physical properties required for a spinal fixation rod(i.e. tensile strength, elastic modulus, proof stress, fatigue strength,and the like), and if the spinal fixation rod also has an elongationgreater than or equal to 15%, then the spinal fixation rod can be saidto have a quite excellent balance of physical properties.

The inventors of the present invention found that, by swaging TNTZ alloyat a cross-sectional reduction rate of 90% and, then, by performingaging treatment at a temperature of at least 700K, it was possible toimprove fatigue strength without a loss of the basic physical propertiesrequired for the spinal fixation rod, and in comparison to agingtreatment at a lower temperature, it was simultaneously possible todramatically improve elongation by performing the aging treatment at thetemperature of at least 700K. The inventors of the present inventionalso found that, even after performance of such extreme working, theother basic physical properties required for a spinal fixation rod weresufficiently maintained, i.e., tensile strength and proof strength (0.2%proof strength). Based on these discoveries, the inventors of thepresent invention conceived of the use of the manufactured TNTZ alloyworked in this manner as a spinal fixation rod and conceived of a spinalfixation system using this type of TNTZ alloy.

Fatigue strength, tensile strength, proof strength, Young's modulus, andelongation of the TNTZ alloy of the present invention were tested in thebelow described manner.

EXPERIMENT 1 EXAMPLES 1 AND 2, COMPARATIVE EXAMPLE 1

Table 1 shows the composition (percent by weight) of the hot forgedround rod prepared as the test sample. This test sample had not beensubjected to solution heat treatment. Thereafter, this test sample wasswaged at room temperature so that the cross-sectional reduction ratebecame 91%.

TABLE 1 Test sample (percent by Nb + weight) Nb Ta Zr O N 0.8Ta TiSW-rod 29.9 13.3 4.73 0.110 0.008 40.54 Remainder

Thereafter, the above-described test sample was subjected to agingtreatment by maintenance in a vacuum for 72 hours (259.2 ks) at 400° C.(673K) during example 1 or at 450° C. (723K) during example 2. Thesample was thereafter cooled using water. The rod obtained duringexample 1 (673K) is referred to below as the SW-rod_(673K), and the rodobtained during example 2 (723K) is referred to below as theSW-rod_(723K). The sample from comparative example 1, which had notundergone post-swaging aging treatment, is referred to as theSW-rod_(as).

COMPARATIVE EXAMPLES 2, 3, 4, 5, AND 6

Table 2 shows the composition of the hot forged round rod preparedduring comparative example 2. This test sample was solution heat treatedby being held for 1 hour (3.6 ks) in a vacuum at 790° C. (1,063K),followed by water cooling.

During comparative example 3, after solution heat treatment in the samemanner as during comparative example 2, aging treatment was performed bymaintenance of the sample for 72 hours (259.2 ks) in a vacuum at 400° C.(673K), followed by water cooling. During comparative example 4, aftersolution heat treatment in the same manner as during comparative example2, aging treatment was performed by maintenance of the sample for 72hours (259.2 ks) in a vacuum at 450° C. (723K), followed by watercooling.

During comparative example 5, after solution heat treatment in the samemanner as during comparative example 2, the test sample was subjected toaging treatment by maintenance for 72 hours (259.2 ks) in a vacuum at400° C. During comparative example 6, after solution heat treatment inthe same manner as during comparative example 2, the test sample wassubjected to aging treatment by maintenance for 72 hours (259.2 ks) in avacuum at 450° C.

The titanium alloy rods obtained during comparative examples 2, 3, and 4are referred to below as ST-rod_(as), ST-rod_(673K), and ST-rod_(723K),respectively. Comparative examples 5 and 6 are referred to below asCR-rod_(673K) and CR-rod_(723K), respectively.

TABLE 2 Test sample (percent by Nb + weight) Nb Ta Zr O N 0.8Ta TiST-rod 30.5 13.0 4.81 0.078 0.09 40.9 Remainder

COMPARATIVE EXAMPLE 7

A Ti-6Al-4V ELI alloy rod (referred to as the Ti64-rod) actually used asa spinal fixation rod was prepared.

EVALUATION OF EXAMPLES AND COMPARATIVE EXAMPLES

The detailed composition of each sample from each of the rods obtainedduring examples 1 and 2 was observed by optical microscopy and fieldemission scanning electron microscopy (FE-SEM). The constituent phasesof each sample were identified using X-ray diffraction (XRD). Duringevaluation of mechanical characteristics, Young's modulus was measuredby the free resonance method, and tensile testing and fatigue testingwere both carried out at room temperature in air. Tensile testing wasperformed using an Instron (R) type tensile tester at a cross head speedof 8.33×10⁻⁶ m/sec. Fatigue testing was performed using a hydraulic typefatigue tester, 0.1 stress ratio, and 10 Hz frequency.

Results of optical microscopic observation of structure are shown inFIG. 2. Equiaxed grains were not found for SW-rod_(as), SW-rod_(673K),and SW-rod_(723K) (comparative example 1 and examples 1 and 2,respectively), which had been subjected to swaging at room temperatureto reach a 91% cross-sectional reduction rate, and each of these wasfound to have a marble-like structure.

The fine structure observation results using FE-SEM are shown in FIG. 3.In SW-rod_(673K) (example 1), a sub-granular structure was found to haveformed along the marble-like structure shape. An extremely fineneedle-like precipitated phase (α phase) was found therein. InSW-rod_(723K) (example 2), a sub-granular structure was also found tohave formed along the marble-like structure shape. Although an extremelyfine needle-like precipitated phase (α phase) was also found, the sizeof this precipitated phase was large in comparison to that ofSW-rod_(673K) (example 1).

FIG. 4 shows results of measurement of Young's modulus by the freeresonance method. Although only the α phase was found to haveprecipitated in both SW-rod_(673K) (example 1) and SW-rod_(723K)(example 2), a high density of dislocations were introduced by swaging,and this is thought to have promoted the precipitation of the α phaseduring these examples. In both SW-rod_(673K) (example 1) andSW-rod_(723K) (example 2), Young's modulus was about 90 GPa, and acomparatively large increase in Young's modulus was found. However, incomparison to the Young's modulus (about 110 GPa) of the Ti-6Al-4V rod(comparative example 7), both of these TNTZ rods maintained a lowervalue Young's modulus. The ability to control Young's modulus at a lowervalue than that of Ti-6Al-4V despite the TNTZ rod having undergoneextreme working (i.e. swaging at a cross-sectional reduction rate of atleast 90% and aging treatment at a temperature of at least 700K) isimportant in that the TNTZ rod can then be used as a spinal FIXATIONrod.

Results of tensile testing are shown in FIG. 5.

In comparison to ST-rod_(673K) (comparative example 3), SW-rod_(673K)(example 1) had a large improvement in tensile strength despite havingsimilar elongation. In comparison to ST-rod_(723K) (comparative example4), SW-rod_(723K) (example 2) had a large improvement in tensilestrength despite having similar elongation. Tensile strength for theSW-rod_(673K) (comparative example 3) and SW-rod_(723K) (comparativeexample 4) favorably compared with that of the Ti64-rod (comparativeexample 7). Based on these tensile test results, SW-rod_(673K)(experiment 1) and SW-rod_(723K) (experiment 2) can be said to have anexcellent balance of strength and ductility.

Fatigue testing results are shown in FIG. 6. Fatigue strength is thevalue of the maximum cyclic stress at 10⁷ cycles of repeated stress.Fatigue strength of the Ti64-rod (comparative example 7) is indicated bythe bold-continuous-line curve and had a value of about 750 MPa. Fatiguestrengths of SW-rod_(673K) (example 1) and SW-rod_(723K) (example 2)were indicated by non-bold continuous-line curves and had values ofabout 850 MPa and about 950 MPa, respectively. Therefore, fatiguestrengths of the SW-rod_(673K) (example 1) and SW-rod_(723K) (example 2)were both found to be higher than that of the Ti64-rod (comparativeexample 7). Fatigue strength values of comparative examples 3-6 were ator below those of comparative example 7.

Table 3 shows results of the evaluations of examples 1 and 2 andcomparative examples 1-6. Tensile strength, elastic modulus, and fatiguestrength of examples 1 and 2 and comparative examples 1-6 are indicatedin comparison to comparative example 7.

TABLE 3 Working Tensile Elastic Fatigue Composition method strengthmodulus strength example 1 Ti—29Nb—13Ta—4.6Zr swaging + aging high lowHigh example 2 Ti—29Nb—13Ta—4.6Zr swaging + aging high low Highcomparative Ti—29Nb—13Ta—4.6Zr swaging approximately low example 1 samecomparative Ti—29Nb—13Ta—4.6Zr solution heat low low example 2 treatmentcomparative Ti—29Nb—13Ta—4.6Zr solution heat approximately low Lowexample 3 treatment + same aging comparative Ti—29Nb—13Ta—4.6Zr solutionheat approximately low Low example 4 treatment + same aging comparativeTi—29Nb—13Ta—4.6Zr solution heat high low Low example 5 treatment +rolling + aging comparative Ti—29Nb—13Ta—4.6Zr solution heat high lowLow example 6 treatment + rolling + aging comparative Ti—6Al—4V standardstandard standard example 7 *plate-shaped test piece

According to the above listed results, examples 1 and 2 had excellentfatigue strength, and elastic modulus is understood to have been lowerthan that of comparative example 7. Moreover, because fatigue strengthof examples 1 and 2 was higher than that of comparative examples 3 and4, it is understood that the combination of swaging and aging treatmentwas good for processing titanium alloy. Because fatigue strength ofexamples 1 and 2 was higher than that of comparative examples 5 and 6,it is understood that swaging was better than rolling as a cold workingmethod for titanium alloy. Although solution heat treatment was notperformed during examples 1 and 2, even if solution heat treatment hadbeen performed prior to swaging, in a manner similar to that of theabove-described examples, it is anticipated that a TNTZ rod could beobtained with a higher fatigue strength than that of the Ti64-rod.

EXPERIMENT 2

TNTZ alloy was manufactured with a Nb content of 29 percent by weight,Ta content of 13 percent by weight, and Zr content of 4.6 percent byweight, with the remainder as Ti. This TNTZ alloy was swaged at 75% or90% cross-sectional reduction rates. The TNTZ alloy swaged at the 75%cross-sectional reduction rate was designated as As-SW75, and the TNTZalloy swaged at the 90% cross-sectional reduction rate was designated asAs-SW90. Thereafter, these swaged TNTZ alloys were subjected to agingtreatment for 259.2 ks at a temperature of 673K and 723K. The 75%cross-sectional reduction rate-swaged TNTZ alloy subjected to agingtreatment at 673K for 259.2 ks was designated as “SW75 aged at 673K for259.2 ks.” The 90% cross-sectional reduction rate-swaged TNTZ alloysubjected to aging treatment at 723K for 259.2 ks was designated as“SW90 aged at 723K for 259.2 ks.” Results of measurement of tensilestrength, 0.2% proof stress, and elongation for the TNTZ alloys obtainedin this manner are shown in FIG. 7 through FIG. 8. FIG. 7 shows tensileproperties of TNTZ sample subjected to aging treatment after coldswaging as functions of reduction ratio and aging temperature. In FIGS.7 and 8, “Elongation” means percentage elongation after fracturemeasured in accordance with JIS Z 2241 (Method of tensile test formetallic materials) of “0.2% Proof stress” means proof stress calculatedby offset method as described in the JIS Z 2241.

As may be understood from FIG. 7 through FIG. 8, the samples of TNTZalloy that had been swaged at 75% and 90% cross-sectional reductionrates but had not been subjected to aging treatment (i.e. As-SW75 andAs-SW90, respectively), had tensile strength values that were both nogreater than 900 MPa, and 0.2% proof stress values were both no greaterthan 800 MPa. However, elongation values for As-SW75 and As-SW90 wereboth at least 15%. Looking at elongation values alone, As-SW75 andAs-SW90 both displayed values of at least 15%, and there appeared to beno deterioration of toughness at the bent part of the rod. However,fatigue strength of the TNTZ alloy was low if the alloy had not beensubjected to post-swaging aging treatment and, thus, As-SW75 and As-SW90would be difficult to use as materials for a spinal fixation rod.

When the TNTZ alloy rods swaged at the 75% and 90% cross-sectionalreduction rate were subjected to aging treatment by heating to 673K or723K for 259.2 ks, elongation was low when the heating temperature was673K. However, when these rods were heat treated at a heatingtemperature 50K higher (i.e. at 723K), it was determined that elongationwas much higher for both the 75% and 90% cross-sectional reduction rateTNTZ alloy rods.

From the standpoints of tensile strength, 0.2% proof stress, andelongation, the best balance of physical properties is displayed in FIG.7-FIG. 8 for the TNTZ alloy rods that had been subjected to swaging at90% cross-sectional reduction rate followed by aging treatment byheating for 259.2 ks at 723K.

When swaging is performed at a cross-sectional reduction rate of atleast 90% and then aging treatment is performed by heating for 259.2 ksat 723K, it is understood from the results of experiment 1 andexperiment 2 that excellent physical properties are provided for aspinal fixation rod. It is understood that such physical properties areequivalent or better than those of the conventionally used Ti-6Al-4Vwith respect to each of the properties of fatigue strength, tensilestrength, 0.2% proof stress, and elongation.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A method for producing a rod made of titaniumalloy, the method comprising: forming a cylindrical round rod of thetitanium alloy, wherein the titanium alloy comprises: Nb content of 25to 35 percent by weight, Ta content of such that the Nb content +0.8×Tacontent ranges from 36 to 45 percent by weight, Zr content of 3 to 6percent by weight, and the remainder is Ti and unavoidable impurities,excluding vanadium; subjecting the cylindrical round rod made of thetitanium alloy to a swaging processing of a cross-sectional reductionrate greater than or equal to 90%; and aging the swaged titanium alloyrod by heating at a temperature of 700 to 800K for 43.2 ks to 604.8 ks.2. The method according to claim 1, further comprising machining theaged titanium alloy rod into a cylindrical shape having a diameter of 4to 7 mm.
 3. The method according to claim 2, wherein the machining ofthe aged titanium alloy rod is performed such that the rod has asufficient length for coupling with spinal fixation screws embedded andfixed in vertebrae of a human body.
 4. The method according to claim 1,wherein the titanium alloy rod has properties under JIS of: a) tensilestrength is greater than or equal to 1,150 MPa, b) fatigue strength isgreater than 900 MPa, c) elastic modulus is less than 110 GPa, d) 0.2%proof stress is greater than 1,000 MPa, and e) percentage elongationafter fracture is greater than or equal to 15%.